High voltage iii-nitride semiconductor devices

ABSTRACT

A III-N device is described has a buffer layer, a first III-N material layer on the buffer layer, a second III-N material layer on the first III-N material layer on an opposite side from the buffer layer and a dispersion blocking layer between the buffer layer and the channel layer. The first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer. A sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patent application Ser. No. 12/465,968, filed May 14, 2009, the disclosure of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This invention relates to semiconductor devices fabricated on group III-nitride semiconductors.

BACKGROUND

Group III-nitride based devices have many potential material advantages over silicon based devices for high power electronics applications. Amongst others, the advantages can include a larger band gap and breakdown field, high electron mobility in a two dimensional electron gas (2DEG) and low thermal generation current. However, large native substrates for group III-nitride semiconductors are not yet widely available. Currently, III-nitride films are grown by heteroepitaxy on suitable non-III-nitride substrates.

Substrates commonly used to support III-nitride films are silicon carbide, sapphire or silicon. Heteroepitaxy can be done with molecular beam epitaxy (MBE) or metal oxide chemical vapor deposition (MOCVD), and lately with hydride vapor phase epitaxy (HYPE). It can be difficult to grow high quality thick gallium nitride layers by heteroepitaxy; therefore, high voltage devices in gallium nitride are typically lateral devices with gallium nitride layers that are only a few microns thick. It can be difficult to accommodate a large voltage in a lateral device without spacing out the electrodes a relatively large distance. For a large blocking voltage across the source/gate and drain in a FET or across the anode and cathode in a diode, the spacing between the electrodes needed to sustain the voltage can be large. For example, a 1 kV device may have gate-drain electrode spacing of 10 μm or larger. This can cause high voltage lateral devices to have a larger area than an equivalent vertical device. Thus, substrate cost becomes an important issue commercially.

To reduce cost, silicon is the most desirable substrate for III-nitride layers. However, due to the large lattice and thermal mismatch between silicon and gallium nitride, it can be necessary to include nucleation and stress management layers in the device structures. These layers, often called the buffer layer and comprised of layers of Al_(x)Ga_(1-x)N that can include superlattices, can have a high density of threading dislocations and other extended defects, along with a multitude of point defects that act as deep traps or dopants. An appropriate buffer layer, even for a lattice mismatched substrate, can result in films of acceptable quality above the buffer layer. However, the layers within the buffer layer can have a high concentration of defect levels in the bandgap. The bandgap defect levels can cause dispersion or current collapse due to electron trapping in these layers, leakage at high drain biases due to carrier generation in these layers, and reduce the breakdown voltage of the device.

FIGS. 1( a) and 1(b) show an approach used to confine electrons to the channel. The III-nitride stack of FIG. 1( a) is for n-channel devices on the cation face, the preeminent form of III-nitride structure currently being used for fabricating HEMTs. The stack can be used to form a lateral device in which external biases modulate field and current in the active layer. The device structure includes a substrate 101 on which a buffer layer 102, which may include nucleation and stress management layers, is grown by heteroepitaxy. The active layer, which includes a channel layer 103 that has a 2DEG 104, is on the buffer layer 102. A barrier layer 105 whose dipole charge enables the formation of the 2DEG and confines the electrons to the channel layer is on an opposite side of the channel layer 103 from the buffer layer 102. Insulation and metallization layers are deposited and patterned to form the device (not shown).

Referring to FIG. 1( b), there can be a step in the conduction band edge (ΔE_(C)) going from the channel layer to the buffer layer. As shown in the band diagram along the plane YY₁, a higher conduction band edge in the buffer layer can prevent carrier injection and trapping in the buffer layer as long as the barrier height is larger than the energy of the electrons impinging on it. Electron e₁ with energy less than the barrier height gets reflected back at the barrier (schematic trajectories r₁ and r′₁) while electron e₂ with energy greater than the barrier gets injected into the barrier where it could get trapped (schematic trajectory r′₂) or get collected by the substrate contact on the other side of the barrier (schematic trajectory r″₂). FIG. 1( b) only shows trapping processes in the buffer layer. However, the defects that form deep levels in the buffer layer also diffuse into the channel layer where they can readily trap electrons and cause current collapse.

SUMMARY

In one aspect, a III-N device is described that has a buffer layer, a first III-N material layer on the buffer layer, a second III-N material layer on the first III-N material layer on an opposite side from the buffer layer and a dispersion blocking layer between the buffer layer and the channel layer. The first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer. A sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer.

In another aspect, an integrate circuit is described. The circuit includes a III-N device, where the III-N device is an enhancement mode transistor or a depletion mode transistor and is electrically connected to one of a diode, resistor or capacitor.

Embodiments of the device may include one or more of the following features. The lowest conduction band minimum in the dispersion blocking layer can be higher than the highest occupied conduction band minimum in the channel layer. The dispersion blocking layer can be configured to confine electrons to the channel layer during device operation. The device can include a spacer layer between the dispersion blocking layer and the buffer layer. The dispersion blocking layer can have a greater concentration of aluminum than the spacer layer. The channel layer can consist of a first III-nitride alloy and the spacer layer consists of a second III-nitride alloy, wherein the first III-nitride alloy and the second III-nitride alloy have aluminum mole fractions within 10% of one another. The material of the channel layer can be unintentionally doped and the spacer layer can be compensated or over compensated.

The spacer layer can consist of a III-nitride ternary alloy. The spacer layer can consist of Al_(x)Ga_(1-x)N, 0≦x≦0.3. The dispersion blocking layer can be a ternary III-nitride alloy layer with a sheet or layer of negative polarization charge adjacent to the channel layer. The dispersion blocking layer can comprise Al_(x)In_(y)Ga_(1-x-y)N, y<x and 0<(x+y)<1. A portion of the dispersion blocking layer that is closer to the channel layer than the buffer layer can have a higher aluminum composition than a portion of the dispersion blocking layer that is closer to the buffer layer. The dispersion blocking layer can have a graded aluminum concentration. The dispersion blocking layer can have a stepped aluminum concentration. The composition of the ternary III-nitride alloy layer can be graded and the ternary III-nitride alloy layer can be intentionally compensated. Fe, C, Mg, Zn or Be or any combination of acceptor or amphoteric dopants can compensate the III-nitride layer. The device can include a spacer layer that is doped with Fe, C, Mg, Zn or Be or any combination of acceptor or amphoteric dopants. The device can include a source contact, a drain contact and a gate, wherein the gate is adjacent to the second III-N material layer, the source contact and drain contact are in electrical contact with the 2DEG and the device is an enhancement mode FET. The device can include a source contact, a drain contact and a gate, wherein the gate is in contact with the second III-N material layer, the source contact and drain contact are in electrical contact with the 2DEG and the device is a depletion mode FET. The device can include an anode that forms a Schottky barrier with the III-nitride stack and a cathode in electrical contact with the 2DEG, wherein the device is a diode. When in operation, the device can have an on-resistance increase under switching operation at voltages above 300V that is less than 10%. In operation the device can have an on-resistance increase that is less than 5%. In operation the device can have operation has an on-resistance increase that is less than 2%. In operation the device can have an on-resistance increase under switching operation at voltages above 1200V that is less than 5%. The channel layer can have a thickness less than 1 micron, such as less than 0.5 micron or less than 0.05 micron. A combined thickness of all III-N layers can be about 2 micron or less and the device can exhibit less than 20% dispersion when used in an application where the device blocks at least 300V. A combined thickness of all III-N layers can be about 2.5 microns or less and the device can exhibit less than 20% dispersion when used in an application where the device blocks at least 600V. A combined thickness of all III-N layers can be about 3 microns or less and the device can exhibit less than 20% dispersion when used in an application where the device blocks at least 1200V. The device can include a substrate on an opposite side of the buffer layer from the dispersion blocking layer, wherein the substrate comprises either silicon carbide, sapphire or substantially pure silicon.

One or more of the embodiments described herein may provide one of the following advantages. One solution to prevent current collapse caused by traps in the buffer layer is to grow a thick (>2 um) GaN channel layer to separate the electrons in the 2DEG from the defects in the buffer. However, while this solution can be a suitable approach for transistors grown on silicon carbide or sapphire substrates, it can be difficult to grow thick uninterrupted gallium nitride layers on silicon substrates. Therefore, the devices and methods described herein do not require a thick channel layer. Rather, channel layers with a thickness of less than 0.5 microns, such as less than 0.2 micron can be utilized. In addition to trapping and current collapse, another major challenge in the fabrication of GaN devices is the creation of insulating buffer layers. One or more types of an intentional impurity such as iron (Fe), carbon (C), or magnesium (Mg) may be added to the buffer to compensate unintentional impurities in order to render the buffer semi-insulating. However, the use of intentional impurities must be managed carefully and balanced with the need to reduce current collapse. For example, a standard AlGaN/GaN transistor can be grown in which the entire GaN layer is doped, for example with Fe, except for a thin portion near the AlGaN/GaN interface where the 2-DEG forms. This structure can result in excellent off-state leakage behavior, but can suffer from current collapse as a result of channel charge being trapped by deep levels associated with Fe during device operation. On the other end of the spectrum, an AlGaN/GaN structure can be grown where no intentional Fe doping is used. This structure will have little current collapse behavior, but has high off-state leakage at high voltages.

The solutions provided herein avoid problems occurring in other III-nitride devices, such as problems that occur in devices that offer other solutions, such as merely forming back barriers in cation faced III-nitride devices to confine carriers to the vicinity of the 2DEG. Some back barriers can result in devices exhibiting high dispersion, because the GaN channel layer next to the AlN/AlGaN buffer layer is not of a very high quality and can therefore trap channel charge during device operation. To prevent defects from forming near the device channel, the buffer layer has to be made thick (>0.5 μm). However, a thick high bandgap buffer layer can act as a poor thermal conductor to the substrate, which is undesirable for power devices. Furthermore, a thick AlN/AlGaN buffer layer with the large Al mole fraction (>20%), required to form the barrier to prevent electron entry into the buffer layer, is more difficult to grow. Even if one grows a thick AlGaN layer with Al mole fraction greater that 20%, it is very defected and causes a defected channel layer to be subsequently formed on it. Indium based quaternary III-nitrides produce a small conduction band discontinuity that acts as a back barrier. These schemes can be used in low-voltage applications to improve channel carrier confinement and thence transconductance and output resistance. However, they have limited applicability for high voltage devices, where operating voltages are greater than 100V, and carrier heating causes electrons to cross these small barriers.

The devices and methods described herein allow for fabrication of III-N devices on various substrates in which dispersion due to buffer layer traps is mitigated. By preventing the trapping of channel electrons in the bulk of the buffer layer, the background doping in the buffer layer can be much higher than would otherwise be practical, leading to a more manufacturable structure. Furthermore, if the channel layer is thin enough, the dopant levels can remain high in the channel layer without significant effect on electrical performance. This allows for the use of dopants (such as Fe in MOCVD) which may “ride” the surface during growth and result in long tails after the dopant source is shut off. The buffer layer is sufficiently insulating. The entire III-N material structure is also sufficiently thin to prevent defects resulting from thermal mismatch between the substrate and the III-N materials from forming. The devices described herein can be used in high voltage applications because they exhibit low trapping and low leakage. They can also be used for high frequency HEMTs to reduce dispersion. By separating the channel from the buffer, the design of optimum buffer layers can be decoupled from the design of the channel structure, resulting in higher performance and a more controllable, repeatable manufacturing process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1( a) shows a III-nitride stack.

FIG. 1( b) is a schematic of a back barrier in GaN based devices showing how a conduction band back barrier can prevent carriers from the channel from getting trapped in the buffer layer.

FIG. 2 shows the general structure of a semiconductor stack with a dispersion blocking layer included.

FIG. 3 shows a cross-section of the semiconductor stack, wherein a dispersion blocking layer is added using a graded AlGaN layer.

FIG. 4 shows a simulated band diagram for a semiconductor stack with a dispersion blocking layer.

FIGS. 5( a) and 5(b) show band diagrams that show how the dispersion blocking layer prevents carrier trapping.

FIGS. 6( a) and 6(b) show the CV and carrier profiles for two semiconductor devices with different compensating schemes below the dispersion blocking layer.

FIGS. 7( a) and 7(b) have graphs showing drain leakage current vs. drain bias when the HEMTs are biased OFF for devices with and without the dispersion blocking layer.

FIGS. 8( a) and 8(b) have graphs of the ON resistance and drain current as a function of the drain voltage for devices with and without the dispersion blocking layer.

FIG. 9 shows the schematic cross-section of a semiconductor stack with a dispersion blocking layer on a silicon substrate with a HEMT fabricated on that.

DETAILED DESCRIPTION

Device structures in which a dispersion blocking layer is included between the channel layer and the buffer layer of a III-N semiconductor device confine carriers to the channel layer so that dispersion or current collapse due to trapping in the buffer layer is minimized. The term dispersion blocking layer describes a layered structure in a III-N device that reduces electron trapping in the buffer layer and thence reduces drain current dispersion or drain current collapse and output resistance increase. The terms dispersion and current collapse are used synonymously herein for drain current reduction due to electron trapping. Additionally, the buffer layer can be made sufficiently insulating, such as by intentional doping, to prevent buffer leakage. Dispersion blocking layers can alleviate the need to grow thick channel or buffer layer(s) in order to reduce trapping and current collapse. The blocking layer can further eliminate the need to optimize the trade-off between reducing current collapse and having a low leakage buffer. By inserting a polarization engineered dispersion blocking layer close to the electron channel, electrons are prevented from entering the defective buffer layer, permitting the insulating nature of the buffer layer to be improved. A dispersion blocking layer can be used with GaN devices formed on silicon.

GaN-based materials and foreign substrate materials such as silicon have different lattice constants and different coefficients of thermal expansion. These differences can be addressed in part by using AlGaInN materials in the buffer layer, which may comprise one or more layers for nucleation, stress management and defect reduction, to offset the effect of the lattice mismatch and the effect of the differences in the coefficient of thermal expansion. In addition, for high voltage devices (V_(DS)>300V, 600V or 1200V, depending on the application) GaN-based epitaxial material grown on a silicon substrate often has a thickness large enough to prevent breakdown of the device fabricated from the GaN epitaxial material. That is, it is desirable to prevent breakdown along the path from the drain to the silicon to the source. This path is possible because the Si substrate has a higher conductivity and lower breakdown strength than the GaN grown thereon.

The various III-N alloys used to form nucleation and buffer layers to manage the effects of lattice mismatch and the coefficient of thermal expansion are typically very defective because of the lattice mismatch and buffer layer's requirements to accommodate strain due to thermal coefficient mismatch. The channel material grown above the nucleation and buffer layer on which the active high power device is fabricated can be grown as pure and thick as possible to attempt to separate the active channel region in the channel layer from the buffer layer. A thick channel layer can reduce the problems of dispersion and current collapse due to electron trapping in defects. However, the maximum thickness of the high purity III-N channel is limited because of the lattice mismatch and coefficient of thermal expansion problem. Further, a thick channel can lead to excessive bowing of the wafer and can potentially cause the wafer to crack. The use of the III-N dispersion blocking layer can mitigate the need for a thick channel, because the III-N dispersion blocking layer prevents electrons from the channel from interacting with the defects in the buffer layer during device operation. This allows the buffer layer to be designed independent of the channel layer, breaking a design trade-off that can exist. The blocking layer can also allow reduced dispersion operation of high power GaN-based devices fabricated on Si substrates.

Referring to FIG. 2, a substrate 1 is an appropriate substrate, such as silicon carbide, sapphire, silicon, a III-nitride or any other material, for the growth of III-nitride semiconducting layers, including cation-face III-nitride layers, by hetero- or homo-epitaxy. A channel layer 4 and a barrier layer 5 form the active portion of the device, with the bandgap of the barrier layer 5 being larger than that of the channel layer 4. Buffer layer 2 is between the substrate 1 and the channel layer 4. In general, the active portion of the device can include any combination of III-N layers that can be used to form a III-N electronic device, such as a diode, HEMT, HFET, MISHFET, POLFET, or other III-N device. Examples of III-N diode and transistor devices, along with III-N layers which comprise these devices, can be found in U.S. Pat. No. 7,915,643, U.S. Pat. No. 7,795,642, U.S. Pat. No. 7,851,825, U.S. Pat. No. 8,519,438, U.S. Pat. No. 7,898,004, U.S. Pat. No. 7,884,394, and U.S. Patent Publication No. 2009/0072269, filed Sep. 17, 2007, all of which are hereby incorporated by reference.

Buffer layer 2 enables the formation of overlying III-nitride layers. The buffer layer 2 can be a simple layer of a single material or can be formed of multiple layers, such as any combination of Al_(x)Ga_(1-x)N/GaN layers. The buffer layer 2 can be lattice mismatched to the substrate. Ideally, the average lattice constant of the lower face of the buffer layer 2 is matched to the substrate 1 and the average lattice constant of the upper face of the buffer layer 2 is matched to the lattice constant of the channel layer. However, this type of lattice matching is difficult to achieve in a defect-free buffer layer 2. Rather, the lattice constant mismatching that realistically occurs between buffer layer 2 and substrate 1 and between buffer layer 2 and channel layer 4 can result in the formation of dislocations and other extended defects which have deep levels. The buffer layer 2 can be intentionally doped, such as with iron (Fe), carbon (C), or magnesium (Mg), to compensate for unintentional n-type dopants, which can cause buffer leakage. The density of deep levels, which can trap channel charge during device operation, can be further increased by the intentional doping to compensate the unintentional dopants in the buffer layer to make it semi-insulating.

Dispersion blocking layer 3 is inserted between the buffer layer 2 and the channel layer 4. The dispersion blocking layer 3 is a thin, such as, less than 500 nm thick, for example, less than 200 nm thick, and if need be compensated, that is, intentionally doped with p-type dopants, layer of a III-nitride material. The dispersion blocking layer 3 is strained such that it is polarized with respect to the channel material. A sheet of negative polarization charge on its upper face creates a field to confine electrons to the upper regions of the channel layer 4 and prevents the electrons from getting trapped in the buffer layer 2. A band-edge discontinuity at the interface of the channel layer 4 and the dispersion blocking layer 3 such that the conduction band minimum is higher in the dispersion blocking layer 3 further helps to reduce electron injection and trapping.

The dispersion blocker layer 3 is essentially a layer, or a combination of layers, which creates a large field and barrier to isolate electrons from traps in the defective buffer layer 2. One way of isolating electrons is to create a large field perpendicular to the plane of the 2DEG that confines electrons to the part of the channel layer 4 that is substantially trap-free so that trapping is, for example less than 10% of the channel charge, during device operation. The field is created by a sheet or layer of negative charge using polarization or delta doping. The field therefore confines electrons to the top of the channel layer. Electrons are isolated by growing a compensated semi-insulating pseudomorphically strained dispersion blocking layer 3, so that polarization creates a sheet of negative charge just below the channel layer. In some cases, a compensated spacer layer is required between the pseudomorphically strained dispersion blocking layer 3 and the buffer layer.

The dispersion blocking layer 3 can be formed of a III-nitride layer of alloys such as a binary alloy AN, or a ternary alloy such as A_(x)Ga_(1-x)N with 0≦x<1, or a quaternary alloy Z_(y)A_(x)Ga_(1-x-y)N with 0≦(x+y)<1, where A and Z are cationic elements. In some embodiments, these layers are graded from one alloy to the next or the alloys can be stepped within the layer. Two simple examples are: an AlN layer or an Al_(x)Ga_(1-x)N layer, where the latter is graded or stepped in Al composition. The dispersion blocking layer can be an AlGaN/GaN superlattice. The dispersion blocking layer 3 can be intentionally compensated with Fe, Mg, Be, C or Zn or any other suitable dopant or combination of dopants that are able to prevent the formation of a 2DEG or a less confined electron distribution in or below the dispersion blocking layer. The extent of the compensation can be from 0 to 100% depending on the device requirement, the tradeoff being between the advantages of little mobile electrons in or below the dispersion blocking layer 3 and the disadvantage of deep defects due to the intentional compensation doping. The charge due to polarization creates the field that raises the conduction band edge in the portion of the channel layer 4 adjacent to the dispersion blocking layer 3. Furthermore, a III-N spacer layer can be included between the buffer layer 2 and dispersion blocking layer 3 and could be considered part of the dispersion blocking layer 3. In some embodiments, the spacer layer is a III-nitride ternary alloy such as Al_(x)Ga_(1-x)N, where 0≦x<0.3. The spacer layer can be compensated, that is, the right amount of the opposite type of dopant is added to make the semiconductor near intrinsic, or overcompensated, that is, the type of material is changed, such as from an n-type material to a p-type material.

An exemplary device having a dispersion blocking layer is shown in schematic in FIG. 3. Substrate 1 is silicon carbide. Buffer layer 2 includes a thin AlN nucleation layer 21 grown on the SiC substrate 1 and an iron doped GaN layer 22 on the nucleation layer. The iron doped GaN layer can be between about 1 and 10 microns thick, such as about 2.5 microns thick. The dispersion blocking layer 3 has a moderately Fe doped GaN spacer layer 31 with a graded Al_(x)Ga_(1-x)N (x going from 0 to 0.23) layer 32 on the spacer layer. The graded Al_(x)Ga_(1-x)N layer is about 27 nm thick. The channel layer 4 is formed of GaN and has a thickness of less than 500 nm, such as about 50 nm. The barrier layer 5 includes a 0.6 nm AlN layer 51 with a 27 nm Al_(0.28)Ga_(0.72)N layer 52 thereon.

A simulated band diagram for the device shown in FIG. 3 is shown in FIG. 4. The addition of the dispersion blocking layer 3 creates a large field E, which raises the conduction band edge 201 at the bottom face of the channel layer 4. Furthermore, the iron doping can prevent the formation of a 2DEG below the graded AlGaN sublayer of the blocking layer 3. Alternatively, an additional 2DEG or other charge distribution can be present in or below the dispersion blocking layer 3, where the concentration of additional charge can be controlled by adjusting the concentration of iron doping. The band diagrams in FIGS. 5( a) and 5(b) show the difference in the barrier for situations where a device has (FIG. 5( a)) and does not have (FIG. 5( b)) an AlGaN dispersion blocking layer 3. As shown in the band diagrams, adding a graded AlGaN blocking layer prevents electrons from being scattered into the buffer layer 2 where the electrons can be trapped.

FIGS. 6( a)-8(b) show the effects of introducing an AlGaN dispersion blocking layer. As shown by the graphs, doping the spacer layer such as by Fe doping, can prevent or reduce the formation of a 2DEG below the dispersion blocking layer 3. The elimination of a 2-DEG below the dispersion blocking layer 3 can reduce device leakage or early breakdown for some device structures. Further, the dispersion blocking layer 3 can improve device switching performance.

FIGS. 6( a) and 6(b) show capacitance-voltage (CV) plots and carrier profiles of two devices with AlGaN dispersion blocking layers. Each carrier profile has two peaks, the peaks P₁ and P₃ closer to the surface are due to the channel 2DEG and the peaks P₂ and P₄ are deeper in the semiconductor indicate the electron concentration below the dispersion blocking AlGaN layer. The plots in FIG. 6( a) are for a device that has been Fe doped right up to the graded AlGaN layer and the plots in FIG. 6( b) are for a device with Fe doping only in the buffer layer. In the latter case a 2DEG layer is formed below the AlGaN dispersion blocking layer, which is shown by P4 being sharper and higher than P2. A 2DEG below the AlGaN dispersion blocking layer can be problematic at large device biases for some device structures.

FIGS. 7( a), 7(b), 8(a), and 8(b) are performance plots of performance parameters of HEMTs with and without a graded AlGaN dispersion blocking layer. In FIG. 7( a), current voltage (IV) plots for devices without dispersion blocking layers are shown and in FIG. 7( b) the IV plots for devices with dispersion blocking layers are shown. The plots (each plot is for a different device) show the drain leakage current as a function of drain bias with the devices biased OFF. Devices with a dispersion blocking layer (FIG. 7( b)) have on the average smaller leakage currents compared to devices without the dispersion blocking layer (FIG. 7( a)).

In FIGS. 8( a) and 8(b), plots of the on resistance (R_(ON)), when the FET is switched ON after being turned OFF and kept at a certain drain bias, is shown as a function of the drain voltage when the device is kept OFF. Also plotted is the drain current (I_(DS)) used for measuring each R_(ON) and as can be seen the drain currents are around the nominal 1 A value set for these measurements. The R_(ON) after the drain is kept at a certain reverse bias is a measure of trapping that modulates the channel. There is little evidence of trapping in the device with the graded AlGaN dispersion blocking layer (FIG. 8( b)). However, the device without the dispersion blocking layer (FIG. 8( a)) shows a steady increase of trapping effects, exemplified by the increase of R_(ON), as the drain voltage is increased.

FIG. 9 shows a cross-section of a GaN-on-Si depletion mode (D-mode) HEMT that has a dispersion blocking layer. On a silicon substrate 1, a III-nitride buffer layer 2 is grown heteroepitaxially. Next, a dispersion blocking layer 3 either with or without a GaN spacer layer is grown on the buffer layer 2. The channel layer 4 and the barrier layer 5 are grown on the blocking layer 3. A D-mode HEMT is fabricated by patterning and forming ohmic source 6 and drain 9 contacts, depositing a suitable dielectric 8 and appropriately micro-machining the dielectric 8 before depositing the gate 7 and the slant field plate on the dielectric 8. The device is optionally coated with a passivation layer (not shown) that is patterned to open contact holes for wire bonding or flip-chip bonding to bond pads. This D-mode HEMT is illustrated as an example of how a dispersion blocking layer could be used in a planar GaN device. E-mode HEMTs, diodes or even GaN integrated circuits can benefit from such dispersion blocking layers.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the dispersion blocking layer could have different types of buffer and active layers on either side, the buffer layer optimized for the substrate and the III-nitride active layer so that the latter have the requisite strain profile and low defect density, and the active layers themselves optimized for the FET, diode or high voltage integrated circuit fabricated in them. The specific HEMT structure and measurements on it that are presented are meant only to show the efficacy of the dispersion blocking layer. The structures described herein can be grown by epitaxy, such as by MBE or HYPE. Although the term “on” is used in the claims to indicate the relative locations of some of the components, such as layers, there may be one or more intervening layers. When the term “directly on” is used, there is no intervening layer. Accordingly, other embodiments are within the scope of the following claims. 

What is claimed is:
 1. A method of forming a III-N device, comprising: forming a first III-N material layer on a buffer layer; forming a second III-N material layer on an opposite side of the first III-N material layer from the buffer layer, wherein the first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer; and forming a dispersion blocking layer between the buffer layer and the channel layer, the dispersion blocking layer being doped with Fe, Mg, Be, C, or Zn; wherein a sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer; a band-edge discontinuity at the interface of the channel layer and the dispersion blocking layer results in a conduction band edge directly adjacent to the interface being higher in the dispersion blocking layer than in the channel layer; and a conduction band minimum of the dispersion blocking layer is within the dispersion blocking layer and away from the interface.
 2. The method of claim 1, wherein the dispersion blocking layer is configured to confine electrons to the channel layer during device operation.
 3. A method of forming a III-N device, comprising: forming a first III-N material layer on a buffer layer; forming a second III-N material layer on an opposite side of the first III-N material layer from the buffer layer, wherein the first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer; forming a dispersion blocking layer between the buffer layer and the channel layer; and forming a spacer layer between the dispersion blocking layer and the buffer layer; wherein a sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer; a band-edge discontinuity at the interface of the channel layer and the dispersion blocking layer results in a conduction band edge directly adjacent to the interface being higher in the dispersion blocking layer than in the channel layer; and a conduction band minimum of the dispersion blocking layer is within the dispersion blocking layer and away from the interface.
 4. The method of claim 3, wherein the dispersion blocking layer has a greater concentration of aluminum than the spacer layer.
 5. The method of claim 3, wherein the channel layer consists of a first III-nitride alloy and the spacer layer consists of a second III-nitride alloy, wherein the first III-nitride alloy and the second III-nitride alloy have aluminum mole fractions within 10% of one another.
 6. The method of claim 3, wherein the spacer layer consists of Al_(x)Ga_(1-x)N, 0≦x<0.3.
 7. The method of claim 3, further comprising forming a source contact, a drain contact, and a gate, wherein the gate is adjacent to the second III-N material layer, the source contact and drain contact are in electrical contact with the 2DEG channel, and the III-N device is an enhancement mode FET.
 8. The method of claim 3, further comprising forming a source contact, a drain contact, and a gate, wherein the source contact and drain contact are in electrical contact with the 2DEG and the device is a depletion mode FET.
 9. The method of claim 3, wherein the first and second III-N material layers are part of a III-nitride stack, the device further comprises an anode that forms a Schottky barrier with the III-nitride stack and a cathode in electrical contact with the 2DEG, and the device is a diode.
 10. The method of claim 3, wherein the device in operation has an on-resistance increase under switching operation at voltages above 300V that is less than 10%.
 11. The method of claim 10, wherein the device in operation has an on-resistance increase under switching operation at voltages above 1200V that is less than 5%.
 12. The method of claim 3, wherein the channel layer is formed having a thickness less than 1 micron.
 13. The method of claim 12, wherein the channel layer is formed having a thickness less than 0.5 micron.
 14. The method of claim 13, wherein the channel layer is formed having a thickness less than 0.05 micron.
 15. The method of claim 3, wherein a combined thickness of all III-N layers in the device is about 2 μm or less and the device exhibits less than 20% dispersion when used in an application where the device blocks at least 300V.
 16. The method of claim 3, wherein a combined thickness of all III-N layers in the device is about 2.5 μm or less and the device exhibits less than 20% dispersion when used in an application where the device blocks at least 600V.
 17. The method of claim 3, wherein a combined thickness of all III-N layers in the device is about 3 μm or less and the device exhibits less than 20% dispersion when used in an application where the device blocks at least 1200V.
 18. The method of claim 3, wherein the buffer layer is formed on a substrate, the substrate comprising silicon carbide, sapphire, or silicon.
 19. A method of forming a III-N device, comprising: forming a first III-N material layer on a buffer layer; forming a second III-N material layer on an opposite side of the first III-N material layer from the buffer layer, wherein the first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer; and forming a dispersion blocking layer between the buffer layer and the channel layer; wherein a sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer; a band-edge discontinuity at the interface of the channel layer and the dispersion blocking layer results in a conduction band edge directly adjacent to the interface being higher in the dispersion blocking layer than in the channel layer; a conduction band minimum of the dispersion blocking layer is within the dispersion blocking layer and away from the interface; and the dispersion blocking layer comprises Al_(x)In_(y)Ga_(1-x-y)N, y<x and 0<(x+y)<1.
 20. The method of claim 19, wherein a portion of the dispersion blocking layer that is closer to the channel layer than the buffer layer has a higher aluminum composition than a portion of the dispersion blocking layer that is closer to the buffer layer.
 21. The method of claim 20, wherein the dispersion blocking layer has a graded aluminum concentration.
 22. The method of claim 20, wherein the dispersion blocking layer has a stepped aluminum concentration.
 23. A method of forming a III-N device, comprising: forming a first III-N material layer on a buffer layer; forming a second III-N material layer on an opposite side of the first III-N material layer from the buffer layer, wherein the first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer; and forming a dispersion blocking layer between the buffer layer and the channel layer; wherein a sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer; a band-edge discontinuity at the interface of the channel layer and the dispersion blocking layer results in a conduction band edge directly adjacent to the interface being higher in the dispersion blocking layer than in the channel layer; a conduction band minimum of the dispersion blocking layer is within the dispersion blocking layer and away from the interface; the dispersion blocking layer is a ternary III-nitride alloy layer with a sheet or layer of negative polarization charge adjacent to the channel layer; and the composition of the ternary III-nitride alloy layer is graded and the ternary III-nitride alloy layer is doped with Fe, C, Mg, Zn or Be.
 24. A method of forming a III-N device, comprising: forming a first III-N material layer on a buffer layer; forming a second III-N material layer on an opposite side of the first III-N material layer from the buffer layer, wherein the first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer; and forming a dispersion blocking layer between the buffer layer and the channel layer; wherein a sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer; a band-edge discontinuity at the interface of the channel layer and the dispersion blocking layer results in a conduction band edge directly adjacent to the interface being higher in the dispersion blocking layer than in the channel layer; a conduction band minimum of the dispersion blocking layer is within the dispersion blocking layer and away from the interface; and the dispersion blocking layer is doped with Fe, C, Mg, Zn or Be or any combination of acceptor or amphoteric dopants.
 25. A method of forming a III-N device, comprising: forming a first III-N material layer on a buffer layer; forming a second III-N material layer on an opposite side of the first III-N material layer from the buffer layer, wherein the first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer; forming a dispersion blocking layer between the buffer layer and the channel layer; and forming a spacer layer that is doped with Fe, C, Mg, Zn or Be or any combination of acceptor or amphoteric dopants; wherein a sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer; a band-edge discontinuity at the interface of the channel layer and the dispersion blocking layer results in a conduction band edge directly adjacent to the interface being higher in the dispersion blocking layer than in the channel layer; a conduction band minimum of the dispersion blocking layer is within the dispersion blocking layer and away from the interface; the dispersion blocking layer is a ternary III-nitride alloy layer with a sheet or layer of negative polarization charge adjacent to the channel layer.
 26. A method of forming a III-N device, comprising: forming a first III-N material layer on a buffer layer; forming a second III-N material layer on an opposite side of the first III-N material layer from the buffer layer, wherein the first III-N material layer is a channel layer and a compositional difference between the first III-N material layer and the second III-N material layer induces a 2DEG channel in the first III-N material layer; and forming a dispersion blocking layer between the buffer layer and the channel layer; wherein a sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer confines electrons away from the buffer layer; the dispersion blocking layer has a wider bandgap than the channel layer; a conduction band minimum of the dispersion blocking layer is within the dispersion blocking layer and away from the interface; and the dispersion blocking layer is doped with Fe, C, Mg, Zn, or Be.
 27. The method of claim 26, wherein the dispersion blocking layer comprises Al_(x)Ga_(1-x)N.
 28. The method of claim 27, wherein the dispersion blocking layer has a graded Al composition.
 29. The method of claim 26, further comprising a spacer layer between the dispersion blocking layer and the buffer layer.
 30. The method of claim 29, wherein the spacer layer is doped with Fe, C, Mg, Zn, or Be.
 31. A method of forming nitride-based device, comprising: forming a first nitride-based material layer on a buffer layer; forming a second nitride-based material layer on an opposite side of the first nitride-based material layer from the buffer layer, wherein the first nitride-based material layer is a channel layer and a compositional difference between the first nitride-based material layer and the second nitride-based material layer induces a 2DEG channel in the first nitride-based material layer; and forming a dispersion blocking layer between the buffer layer and the channel layer; wherein the dispersion blocking layer is strained to induce a sheet or a distribution of negative charge at an interface of the channel layer and the dispersion blocking layer; and a conduction band minimum of the dispersion blocking layer is within the dispersion blocking layer and away from the interface.
 32. The method of claim 31, further comprising forming a source contact, a drain contact, and a gate, wherein the gate is adjacent to the second nitride-based material layer, the source contact and drain contact are in electrical contact with the 2DEG channel, and the III-N device is a FET.
 33. The method of claim 31, wherein the dispersion blocking layer comprises Al_(x)Ga_(1-x)N.
 34. The method of claim 33, wherein the dispersion blocking layer has a graded Al composition.
 35. The method of claim 31, further comprising forming a spacer layer between the dispersion blocking layer and the buffer layer.
 36. The method of claim 35, wherein the spacer layer is doped with Fe, C, Mg, Zn, or Be.
 37. The method of claim 31, wherein the dispersion blocking layer is doped with Fe, C, Mg, Zn, or Be. 